Hmm. Maybe we can infer that a bullated leaf has evolved so that at least a portion of each leaf surface is oriented toward the light source in a way that it is achieving maximum photosynthesis? Just a speculation. In my aroids, nearly everything I grow is a swamp plant or a shallow water species (mainly Cryptocoryne). Since there are bullated species and smooth species, it is probably not an effect of humidity. Or maybe bullation serves to passively collect detritus, so as to provide some nutrients? As they say, a single fool can ask more questions than a thousand wise men (or women) can answer.

I think that part of the frustration you're experiencing may be due to the plant's nature not to be mobile - this may initially sound quite absurd, and have little to nothing to do with bullate leaves, but I think that part of the much more sedentary nature of plants is that some evolutionary quirks have come and gone, while others have developed and stayed, even though a specific functionality is not always present, or in some cases is no longer present. When speed and various types of movement are particularly significant to survival, there's more often the case of gradual elimination of traits not crucial. Of course it could be argued the other way - that since there's no movement then the elimination of traits no longer needed in a microhabitat that is drier or higher light would happen more quickly - but of course the randomness of the genetic shift has to come into play as well. I'm really just playing with an idea here, but the idea that bullate leaves might be a response to one thing but then there are exceptions and what would it mean then would be at least partially calmed by the lack an additional evolutionary shift. Just musing on a Friday afternoon.

I'm thinking bullate leaves likely is a functional form that is then further modified by things like cuticle, leaf thickness, etc., and the "bullate"-ness of a leaf may be a term capturing multiple different things. I googled "bullate anthurium" and came up with A. radicans, a species I adore. It is distinctly bullate, but also distinctly happy as a houseplant in my conditions and is not limited to high humidity... but those are some mighty thick leaves with a nice waxy cuticle. The mystery continues!

As for your question on photon capture - I highly recommend the book "Nature's Palette: The science of plant color" by David Lee. I think it will hit a lot on the light functionality stuff you are looking for, and has a lot of cool plant bits in it. And speaking of that book, there are some great sections that go into cells as lenses and leaves as lenses (very cool!) but never directly addresses bullate - which is something in between those two extremes. Likely because bullate leaves (where the whole leaf is folded up like an origami structure) doesn't have as much influence on color (the point of the book) as it does in water and air exchange. There are some clues that it may play a part in light diffusion though... the book was fascinating but also had me asking almost as many questions as it answered :)

It's frustrating to me to be trained to look at a fish and use traits to identify things (highly forked tail for speed, etc.) but I can't find references to be able to learn the same for plants. I seem to be able to find a lot of physiology books that tell me the chemical pathways and how it develops, but not much on the "why" behind these traits.

The reason for bullation is completely unknown to me. I do have several plants with bullated leaves, but they seem to grow (or not grow) regardless of that feature compared with similar species without bullation.

I remember asking this list once about green plant parts and whether or not being green indicates that photosynthesis takes place there. That is, are green stems, petioles, and such non-leaf-blade parts producing any photosynthetic products? I don't recall having seen an answer to that. It seems that a plant would not bother producing chlorophyll unless they contributed in some way, even if only marginally.

Another mechanical question that I have had since then may bear on bullation. That is, is there a fixed directionality to photon capture? For example, if a leaf gets turned upside down it generally dies if it cannot be righted in time. That suggests to me that even though the underside of the leaf is green, it cannot process light in order to pay its way, so to speak, in the plant's economy. That makes sense to me, but I'd be interested in a source that discusses the issue.

I'm hoping this scientifically savvy group can help me with a topic I've been pondering but cannot seem to find references for. "Bullate" textured leaves seem to occur in a huge range of tropicals (in addition to a number of Aroids, I'm seeing it in Gesneriads, Begonia, Hoffmannia, Piper, Peperomia, etc.). Are they all bullate for the same reasons??

High humidity and increasing air exchange is one simple explanation I've come across a lot (cloud forest species like Anthurium clidemoides for example) and understory plants in consistent humidity seen to fit that (Hoffmannia bullata and some Gesneriads of the Gasteranthus and Nautilocalyx genera), but then the highly bullate Gesneriad Sinningia bullata is confusing... extremely bullate, but in succulent conditions of full sun and wildly swinging temps (which likely is why underneath the leaves looks like a sheep). Even it's highly bullate hybrids with no hairy under leaves do fine in low humidity. Is this a case of adaptation to distribute light on a way to not cook the leaves?

I also read a snippet about bullate possibly being a way to capture more light on low light situations (makes particularly good sense in dark leaved understory plants), but then they full sun Sinningia is confusing again - unless it's using the same idea but different angles to scatter light a different way.

I'd like to dive deeper into this kind of leaf functionally but can't find good resources. Is there a book on plant morphology or function of structures I should be reading?